A semiconductor optical memory which is turned on by a minute trigger light and remains to emit an output light,even after the trigger light ceases is an important key device in image processing systems, optical computers, and the like. Such a semiconductor optical memory has been described on pages 596 to 600 of "J. Appl. Phys. 59(2), Jan. 15, 1986". The semiconductor optical memory comprises a p-GaAs substrate, and a buffer layer of p-GaAs having a thickness of 0.3 .mu.m, an anode layer of p-AlGaAs having a thickness of 0.5 .mu.m, an n-gate layer of n-GaAs having a thickness of 1 .mu.m, a p-gate layer of p-GaAs having a thickness of 50 .ANG., a cathode layer of n-AlGaAs having a thickness of 1 .mu.m, and a cap layer of n-GaAs having a thickness of 0.5 .mu.m grown successively on the first surface of the p-GaAs substrate. The semiconductor optical memory further comprises a p-electrode provided on the second surface of the p-GaAs substrate, and an n-electrode provided on the cap layer.
In operation, where a voltage which is applied across the p-and n-electrodes of the semiconductor optical memory in the forward direction is increased without a trigger light supplied thereto, the semiconductor optical memory is turned on at a predetermined voltage which is defined as a switching voltage. Then, an output light is emitted from the semiconductor optical memory which is shifted from a high impedance state to a low impedance state. In the low impedance state, carriers are injected into the n-gate layer, so that the carriers are confined in a potential barrier defined by the anode and cathode layers having large bandgap energies. This results in a light emission with a high efficiency. In this circumstance, the carriers are mainly confined in the n-gate layer of a larger thickness, and light is also confined dominantly in the n-gate layer in AlGaAs/GaAs system semiconductor memory, because AlGaAs is lower in refractive index due to the inclusion of Al. Even if the applied voltage is decreased below the switching voltage, the low impedance state is held, unless the applied voltage is decreased below a holding voltage.
On the other hand, a trigger light having a predetermined level is supplied to the semiconductor optical memory which is thereby turned on to emit an output light continuously, where a bias voltage which is lower than the switching voltage by a predetermined small voltage is applied across the p-and n-electrodes of the semiconductor optical memory. This is a function of an optical memory. The trigger light is mainly absorbed in the n-gate layer, wherein a wavelength of the trigger light must be shorter than a wavelenghth of GaAs bandgap.
In the above described semiconductor optical memory, a carrier concentiation of the p-gate layer is 1.times.10.sup.19 cm.sup.-3, and that of the n-gate layer is 1.times.10.sup.17 cm.sup.-3. Where a voltage applied across the p-and n-electrodes is increased in the forward direction in the high impedance state, a reverse bias voltage is applied across the p-and n-gate layers, so that a depletion layer is extended dominantly into the n-gate layer having the lower carrier concentration. In this circumstance, when the applied voltage is increased to be the switching voltage, an avalanche breakdown occurs in the depletion layer, so that the semiconductor optical memory is turned on to be in the low impedance state. Generally, an electric field strength, under which an avalanche breakdown occurs, is reversely proportional to an exponent of "3/4" relative to an impurity concentration of a lower concentration layer which is included in a pn junction of one-side step type composed of p-and n-layers of first and second uniform impurity concentrations having a sufficient concentration difference therebetween.
In a practical use, it is preferable to set a switching voltage to be 4 V. For this purpose, it is required to set a carrier concentration of the n-gate layer to be 1.times.10.sup.17 cm.sup.-3, wherein a spontaneous emission life time .tau. is obtained to be 100 ns (.tau.=100 ns). An operation speed of the semiconductor optical memory is determined by the spontaneous emission life time .tau.. Where a rise time and a fall time of an emitted light are 100 ns, respectively, the semiconductor optical memory can operate at the maximum frequency of 10 MHz.
However, the semiconductor optical memory has a disadvantage in that it can not be used in a response speed of several 100 Mb/s by use of a practical switching voltage. As described above, the spontaneous emission life time .tau. is preferable short, thereby increasing a response speed. The life time .tau. becomes short, as the carrier concentration of the n-gate layer is increased. it is very difficult for the carrier concentration of the n-gate layer to be larger than the aforementioned value, because a current-voltage characteristic which is inherent to a pnpn structure is no longer obtained, if it is assumed that the carrier concentration is 1.times.10.sup.18 cm.sup.-3 which is ten times the aforementioned value. As a result of this increase of the carrier concentration, a switching voltage is lowered to be less than a holding voltage which is equal to or less than approximately 1.6 V. For this reason, no semiconductor optical memory which can operate in a response speed of several 100 Mb/s with a practical switching voltage has been proposed.